1. Field of the Invention
The present invention is applied to digital images. In particular, the present invention relates to an image processing apparatus applied to noise reduction processing for images in which a ratio of signal components to noise components (S/N ratio) locally varies, typically medical diagnostic images such as nuclear medicine images, CT images, and MRI images.
2. Description of the Related Art
Noise reduction in digital images has been performed by cutting off (eliminating) high-frequency components by high-frequency cutoff filters, such as Butterworth filters and Gauss filters. However, since the same high-frequency cutoff processing is performed through the whole images, there is caused a problem that part of information is deteriorated in medical diagnostic images such as nuclear medicine images, CT images, and MRI images, although it causes no problem in general digital images (such as landscape images taken by digital cameras).
The first cause of this problem is that a ratio of signal components to noise components (S/N ratio) locally varies from position to position (from pixel to pixel in minimum unit) in medical diagnostic images. This is because the S/N ratio serving as a total noise of a position varies according to collection counts obtained from each pixel in medical diagnostic images. Therefore, when the same high-frequency component cutting processing is performed through the whole image, overcorrected parts and insufficiently corrected parts are generated according to positions. As a result, the processed image includes regions having deteriorated information (such as spatial resolution and contrast) (overcorrection) and regions in which noise reduction is insufficient (insufficient correction).
The second cause of the problem is that medical diagnostic images have a rough (large) pixel size. For example, spatial resolution of nuclear medicine images is about 10 mm, and the pixel size of images expressing it is several millimeters. When filtering is performed by Fourier transform, there are cases where sufficient sampling is not performed and artifacts are generated, since the pixel size is large. Refer to Jpn. Pat. Appln. KOKAI Pub. No. 2001-59872.